Double check the connector for image process unit (Drum/DV) White connector to the left when you open front cover. Error code is a toner density sensor prob, could just be that connector isnt plugged in right. power down before reconnecting!

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1) The Carrier (iron filings) inside the imaging unit is deteriorated and needs to be replaced.

2) The Toner sensor inside the imaging unit may be giving an incoorect reading, due to be faulty or because of old toner/carrier mix blockage close to sensor. Give the imaging unit a good service and replace the carrier.

3) The wires behind the black cover of the imaging unit have been picnhed and is shorting on the metal plate

C710 Toner sensor problem• The sensor output voltage is outsidethe range of 0.5 to 4.5 V during toner control.• The toner sensor control voltage cannot be set within the setting range when maintenance item U130 is run.

Try removing and refitting drum and toner.
if this does not help you may have damaged your toner cartridge or the sensor

To clear this error:
Power the printer off and let it stand for at least 30 minutes to allow the fuser to cool.
Remove the coating roller from the top of the fuser unit to reveal the orange fuser rollers underneath.
Check to see if any paper/transparency has become wrapped around the fuser rollers or jammed in or above the fuser unit itself.
Since some paper jams can be very difficult to see with the fuser installed, remove the fuser and check for paper/transparency jams under the unit as well. A jam in that area can cause 993 errors.
If there is no media jam and the error remains after the printer is restarted, try replacing the Oil Coating Roller (OCR). The OCR comes with the black toner cartridge. (It can also be purchased separately. Click here to do so.)
If the error remains after the OCR has been replaced. If that does not help I would highly recommend taking it back to the place you bought it from